Mercury

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• Mercurys Surface Composition
• Kerri Donaldson Hanna

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Questions answered by studying surface composition

• What type of geologic history has Mercury
  undergone?
• This would constrain the thermal evolution of the
  planet
• How much FeO is on the surface?
• This would constrain the evolution models
  discussed last week
• How much space weathering has occurred on
  Mercurys surface?
• This would constrain the space environment of the
  planet over its history
• Does any of the material in the exosphere come
  from Mercurys surface?
• This would constrain the interactions between the
  exosphere and the magnetic field, solar wind,
  and/or surface

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Common minerals on planetary surfaces

• Feldspars (K,Ba,Ca,Na)Si3O8
• Two groups K - Ba solutions and Ca - Na
  solutions (plagioclase)
• Anorthite most abundant plagioclase on the Moon
• Pyroxenes (Mg,Fe,Ca)(Mg,Fe)Si2O6
• Two groups orthopyroxenes and clinopyroxenes
• Fe-rich, Mg-rich, Ca-rich, NaAl-rich, and
  CaMn-rich
• Olivines (Mg,Fe)2SiO4
• Common in the mantle on Earth
• Solid solution between Mg-rich and Fe-rich
• Fe-Ti Oxides FeO, TiO2, FeTiO3
• Other minerals include sulfates, sulfides,
  carbonates, amphiboles, micas
• On Mercury no plate tectonics or hydrologic
  cycle, should expect rocks and minerals that are
  associated with the crystallization of magma,
  possible igneous intrusions, and meteorite impact
  melting, fracturing, and mixing

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Mercury versus the Moon

• Originally Mercury thought to be similar to the
  Moon
• Bright craters and dark plains
• Smooth plains associated with impact craters and
  basins

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Mariner 10 Observations

• No observations made that could determine
  elemental abundances, specific minerals, or rock
  types on Mercury
• Mariner 10 observed day side albedos of Mercury
  and the Moon
• Dark plains would have a lower albedo than a
  bright crater
• Mercurys albedo lower overall than the Moons by
  a few percent, but in the visible it has a higher
  albedo
• Mercurys albedo varies across its surface and at
  different wavelengths from 400 to 700 nm
• Composition, grain size, and porosity plays key
  roles in explaining a planets albedo
• Finely crystalline silicates low in Fe and Ti
  tend to be brighter and scatter more light off of
  the surface
• New measurements from SOHO paired with Mariner 10
  data looked at phase angle and backscattering
• Results indicate Mercurys surface has smaller
  grains and more transparent than the Moon, and
  the higher efficiency of reflecting light towards
  the sun indicates the presence of complex or
  fractured grains

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Re-calibration of Mariner 10 Images

• Technique first used on lunar data
• Robinson and Lucey 1997
• Use 375nm (UV) and 575nm (VIS) bands
• Ratio UV/VIS
• Plot UV/VIS versus VIS
• As FeO increases and soils mature spectrum
  reddens and UV/VIS decreases
• As opaque minerals increase the albedo decreases
  and increases the UV/VIS
• Rotate axis to decouple FeOmaturity from opaque
  index

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Re-calibration of Mariner 10 Images

• FeO maturity
• Brighter tone indicate decreasing FeO and maturity

VIS image

• UV/VIS image
• Brighter tone indicate increasing blueness

• Opaque Index
• Brighter tone indicate increasing opaque minerals

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Remote Sensing of Planetary Bodies
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Remote Sensing of Planetary Bodies

• Spectroscopy
• Visible light (0.4 - 0.7 ?m)
• Near-IR (0.7 - 2.5 ?m)
• Mid-IR (2.5 - 13.5 ?m)

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Visible to Near-IR spectroscopy

• Measuring reflected light
• Absorption bands are created from electronic
  transitions in the molecules bonded in the
  lattices of silicates
• Interested in 0.3 - 0.5 and 1.0?m bands
  associated with FeO
• Spectral contrast of features can be diminished
  due to space weathering
• Spectral slope - indication of the maturity and
  composition
• Fit straight line from 0.7 - 1.5?m
• Slope of line increases as soil matures
• Look at ratios to determine soil maturity and FeO
  and opaque mineral content
• Again -- techniques used originally on the Moon

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Visible to Near-IR results

• Weak 1?m band detected during 1 observation run -
  only in bright materials
• Shape and width of 1?m band indicative of Ca-rich
  clinopyroxene
• Mercurys spectral slope has a higher value than
  the spectral slope from immature to submature
  regions on the Moon
• Low FeO (0 - 3) and TiO2 (0 - 2)

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Mid-IR spectroscopy

• Measuring emitted light
• Absorption bands are caused by the vibration,
  bending, and flexing modes of the crystalline
  lattices
• Grain size and composition of mineral samples
  greatly affect spectra
• Compare key spectral features diagnostic of
  composition with spectra of rocks and minerals
  measured in the laboratory
• Reststrahlen bands - fundamental molecular
  vibration bands in the region from 7.5 - 11 mm
• Emissivity maxima (also known as the Christensen
  feature) - associated with a silicate spectrum
  and occurs between 7 - 9 mm
• Transparency minima - associated with the change
  from surface scattering to volume scattering and
  occurs between 11 - 13 mm
• Good indicator of SiO2 weight percent in rock
• Highly depends on the quality of spectral
  libraries built from laboratory measurements of
  rocks and minerals

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Diagnostic Spectral Features
CF
RB
TM
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Grain Size and Composition Effects in the Mid-IR

• Varying the grain size changes the depth/or
  existence of spectral features

• Varying the composition changes the location of
  spectral features

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Mid-IR results

• Mercurys surface composition is heterogeneous
• Most spectra match models of plagioclase feldspar
  with some pyroxene
• Plagioclase more sodium-rich than that on the
  Moon
• Pyroxene low-Fe, Ca-rich diopside or augite or
  low-Fe, Mg-rich enstatite
• Bulk compositions indicate an intermediate silica
  content (similar to diorite or andesite on Earth)
• No evidence for Fe- and/or Ti-bearing basalts as
  lava flows as seen on the Moon

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Observing Mercury and the Moon in the mid-IR

• NASA Infared Telescope Facility (IRTF) using
  Boston Universitys Mid-Infrared Spectrometer and
  Imager
• IRTF allows for pointing telescope near the sun
• MIRSI covers the 8 - 14 ?m spectral range
• Mercury - daytime observations
• Moon - day and night time observations
• Locations on the lunar surface with well known
  composition from near-IR telescopic observations
  and Apollo sample returns chosen

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How does a spectrometer work?
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The Moon - Grimaldi Basin
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Grimaldi and Laboratory Spectra Comparison

• Grimaldi spectra compare well in overall shape
  with the RELAB Impact Melt and Breccia spectra
• Grimaldi spectra also compare well with Salisbury
  et al. NoriteH2, in particular 11 13 µm region
• No perfect matches yet, but indicates our results
  are reasonable

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Mercury
250 - 260
200 - 210
175 - 185
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Spectral Deconvolution

• Ramsey (1996) and Ramsey and Christensen (1998)
  developed algorithm and provided in ENVI by Jen
  Piatek
• Inputs spectrum to be deconvolved, spectral
  library of pure mineral spectra, and wavelength
  region to be fit over
• Spectral library of 337 end-members created with
  reflectance spectra of fine and coarse grain
  minerals (ASTER, RELAB, USGS, ASU and BED)
• When minerals in an assemblage are present in
  library, algorithm determines abundances within
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• Previous successes for whole rocks, meteorite
  samples and plagioclase sands include Hamilton
  et al. (1997), Feely and Christensen (1999),
  Hamilton and Christensen (2000), Wyatt et al.
  (2001), and Milam et al. (2007)

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Spectral deconvolution results for Mercury

• Feldspar
• An90-10 (Bytownite - Oligoclase)
• K-spar
• Orthoclase or Sanidine
• Pyroxene
• Hypersthene, Enstatite, and Diopside
• Olivine
• Mg-rich (Fo66-89)
• TiO2
• Rutile
• Small amounts of garnet
• Mg- and Ca-rich garnets

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What do and dont we really know?

• Surface composition heterogeneous
• Feldspar-rich of moderate Ca and Na
• Low-Fe pyroxenes and olivines present
• Low FeO content (up to 3)
• No observations contradict the scenario of early
  core formation accompanied by global contraction
  of the planet
• Extrusive lava flows on the surface are likely
  low in SiO2 and enriched in K and Na
• Not clear about space weathering - different than
  the Moon
• Still not enough info to constrain evolution and
  thermal history models
• Still unsure of link between surface and exosphere

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MESSENGER

• Mercury Atmospheric and Surface Composition
  Spectrometer (MASCS)
• UVVS covers 88.4 254.2 nm
• VIRS covers 216.5 1395.2 nm
• Mercury Dual Imaging System (MDIS)
• 12 filters over the 395 1040 nm spectral range
• Gamma Ray and Neutron Spectrometer (GRNS)
• Will measure cosmic-ray excited elements O, Si,
  S, Fe, and H
• Will measure naturally radioactive K, Th, and U
• X-Ray Spectrometer (XRS)
• Will measure K? lines for Mg, Al, Si, S, Ca, Ti,
  and Fe